The present invention relates to a process for preparing formamide by reaction of ammonia and carbon monoxide.
Formamide (NH2CHO) is an excellent nonaqueous solvent for many inorganic salts (e.g. chlorides of copper, lead, zinc, tin, cobalt, iron, aluminum and nickel; acetates of the alkali metals, etc.). It also dissolves, for example, gelatin, glucose, starch, polyvinyl alcohol and cellulose acetates. Owing to its bifunctionality (carbonyl and amide group), formamide is capable of undergoing numerous reactions in organic chemistry. It reacts with formaldehyde to form a hydroxymethyl derivative, is converted catalytically into acrylonitrile, decomposes at  greater than 200xc2x0 C. into carbon monoxide, ammonia, hydrocyanic acid and water and hydrolyzes very slowly at room temperature, more rapidly at higher temperatures and in the presence of acids or bases.
Formamide is used, in particular, as solvent for producing vitamins, formic acid and hydrocyanic acid, for softening size and paper, as swelling agent in permanent wave fluid, etc.
Formamide is generally prepared from the raw materials carbon monoxide and ammonia. This involves catalytic reactions in which suitable catalysts are, in particular, metal catalysts such as ruthenium catalysts or alkoxides such as sodium methoxide or sodium ethoxide. Processes employed to date which make use of alkoxides as catalyst are two-stage processes in which carbon monoxide and methanol are reacted to give methyl formate in a first stage. The catalyst, usually sodium methoxide, decomposes under the reaction conditions to form sodium formate. This salt generally deposits in the apparatus used which then has to be flushed regularly to free it of salt, which has the disadvantage of loss of production time. In a second stage, the methyl formate reacts with ammonia in the absence of catalysts to produce formamide. Production processes for formamide are usually continuous processes.
In the following we will concern ourselves with a single-stage production process. In GB-A-2 216 035, the reaction conditions are selected so that the solvent remains in the reactor. Use is made of high-boiling solvents (having a boiling point higher than that of formamide) so that formamide can be distilled directly from the reaction mixture and the solvent does not have to be worked up.
Apart from being able to separate off the formamide product easily and being able to regenerate the solvent as simply as possible, there is the additional aim of being able to reuse the catalyst. The catalyst, namely an alkoxide, i.e. a homogeneous catalyst, is normally removed from the reactor in the work-up and is decomposed, for example, by quenching. DE-C-44 33 507 describes the synthesis of formamide using alkali metal ethylene glycolates as catalyst and ethylene glycols as solvent. Owing to the high boiling point of ethylene glycol, which simultaneously functions as a reactant, formamide can readily be distilled from the reaction mixture. In addition, it is possible to recover the catalyst, namely the alkali metal ethylene glycolate, and to reuse it subsequently. However, the high boiling point of the solvent has the disadvantage that it promotes the decomposition of the catalyst.
In the recovery of these alkoxides, it needs to be borne in mind that losses of catalyst occur. This is because, in particular, alkoxides are generally readily hydrolyzed by water. In addition, other secondary reactions take place. Alkoxides can, for example, decompose, which is promoted by relatively high temperatures. A substantial disadvantage of this process is, as mentioned above, the quite low stability of the catalyst used.
It is an object of the present invention to provide a process in which the direct synthesis of formamide from carbon monoxide and ammonia proceeds in such a way that the product can easily be separated from the reaction mixture and, in addition, the catalyst is recovered and reused. It should use a catalyst having a high catalytic activity and a high stability (in particular thermal and chemical stability). It should be possible to recycle the catalyst without problems. During the reaction and work-up, only small losses of the catalyst should occur. In addition, unreacted starting materials as well as solvents should be able to be recovered and be available for reuse in the reaction. A significant aspect is that carbon monoxide should be able to be used not only as a pure substance but also together with hydrogen or other inert gases. Hydrogen therefore has to be inert under the process conditions chosen.
We have found that this object is achieved by a process for preparing formamide by reaction of ammonia and carbon monoxide in the presence of at least one catalyst if sodium diformylamide is used as catalyst.
In a preferred embodiment, the reaction is carried out in the presence of at least one alcohol, preferably in the presence of methanol.
In a further embodiment, the process for preparing formamide by reaction of ammonia and carbon monoxide takes place in the presence of the catalyst sodium diformylamide and a further active component in a reactor to form a formamide-containing reaction mixture and the reaction mixture is subsequently worked up in an apparatus. Here, sodium diformylamide is formed from the active components during the reaction in the reactor and/or during the work-up in the apparatus and sodium diformylamide and/or a mixture containing sodium diformylamide is separated off in the apparatus and fed into the reactor.
In a particularly preferred embodiment, the active component is an alkoxide, preferably sodium methoxide. For the purposes of the present invention, the terms sodium methoxide and sodium diformylamide refer not only to the sodium salts themselves but also to all comparable salts which can form alkoxide or diformylamidexe2x80x94any cation can be chosen in principle. Preferred alkoxides and diformylamides are the corresponding alkali metal and alkaline earth metal salts (for example potassium methoxide or potassium ethoxide). The reaction of ammonia and carbon monoxide proceeds according to the following simplified reaction scheme: 
In the novel process for preparing formamide, the reaction is usually carried out at from 50 to 200xc2x0 C., preferably from 100 to 110xc2x0 C. In general, the reaction takes place at pressures of from 10 to 200 bar. The abovementioned temperatures and pressures are in the reactor in which the reaction takes place.
In the process of the present invention, carbon monoxide can be supplied in the form of a mixture comprising carbon monoxide and hydrogen, preferably synthesis gas. The mixture comprising carbon monoxide and hydrogen is generally then fed into the reactor. It is thus possible to choose to use either pure carbon monoxide or a gas comprising carbon monoxide. Synthesis gas having a high proportion of hydrogen ( greater than 50%) can also be used. This is made possible by the fact that hydrogen is inert under the reaction conditions preferred according to the present invention (in particular the catalyst, the temperature, the pressure and the solvent). Synthesis gas is generally significantly cheaper than pure carbon monoxide, so that its use gives a cost advantage.
In the process of the present invention, the reaction is preferably carried out in the presence of at least one alcohol, preferably in the presence of methanol.
A significant step on the way to discovering the process of the present invention was possibly that it was found that sodium methoxide is converted into sodium diformylamide under the above-described conditions (solvent, temperature, pressure)xe2x80x94sodium diformylamide is thus a downstream product of sodium methoxide. Sodium diformylamide is a significantly more stable catalyst than sodium methoxide. For example, it reacts with water significantly more slowly than does sodium methoxide (is thus less susceptible to hydrolysis).
A further important advantage of sodium diformylamide is that it, in contrast to alkoxides, forms no insoluble salts in the reaction mixture under the prevailing reaction conditions, so that the abovementioned blockage problems do not occur when using sodium diformylamide.
The particular stability of the sodium diformylamide catalyst allows it to be recovered and to be reused in the reaction.
The reaction mixture formed in the reactor is generally passed directly to work-up in the apparatus. In this apparatus, in particular, materials separation by distillation takes place. In addition, a large part of the sodium methoxide transferred to the apparatus (assuming that sodium methoxide is used) is converted into sodium diformylamide in the apparatus. In general, the reaction mixture from the reactor is first separated into liquid and gaseous constituents in a separator and the liquid product is freed of methanol (if methanol is used as alcohol) in a first column and the methanol (if used) is returned to the reactor. Subsequently, after passage through a heat exchanger, the catalyst is usually separated from the formamide product in a second column. However, the catalyst is generally not isolated as a pure substance but is separated off in the form of a mixture comprising sodium diformylamide and formamide, preferably at least 60% by weight of formamide.
The mixture separated off is generally an about 20% strength by weight solution of the catalyst in formamide. While part of this solution is generally bled off in order to prevent accumulation of decomposition products, the other part of this solution is preferably returned to the reactor. Thus, in this continuous production process for formamide, the catalyst is recovered and recycled.
Sodium methoxide is the precursor of the actual sodium diformylamide catalyst; sodium diformylamide is possibly a reaction product of sodium methoxide and formamide. The sum of the concentrations of sodium methoxide and sodium diformylamide in the reactor is usually from 0.05 to 1.0 mol/l, preferably from 0.2 to 0.25 mol/l. Losses of sodium methoxide and/or sodium diformylamide are generally compensated for by metering further amounts of sodium methoxide into the reactor. Preferably, sodium methoxide in the form of a 30% strength by weight methanolic solution is added to the reaction mixture. In principle, it is also possible to meter additional sodium diformylamide into the reactor.
Unreacted carbon monoxide and/or unreacted ammonia and/or alcohol are generally taken from the apparatus and recirculated to the reactor.
The alcohol used is frequently methanol. In this case, the molar ratio of ammonia to carbon monoxide to methanol is generally 1.0:1.0-1.5:0.5-1.5. The ratio 1.0:1.25:1.0 has been found to be particularly useful. The pressure in the reactor is, as described above, preferably from 10 to 200 bar. When using pure carbon monoxide, a pressure of 40 bar is preferred and when using a gas comprising carbon monoxide, for example synthesis gas, a pressure of 120 bar is preferred.
The invention is illustrated by the examples below.